In this post, we discuss the potential measurement errors in sensitive analysis and measurement equipment caused by a mismatch in resistor TCR and how a thick film resistor network may be one possible solution.
Precision resistors are used in a wide range of measurement and analysis equipment. Voltage dividers and precision references are common in such applications with two (or more) resistors utilised to provide a known reference or input voltage
In these applications the ratio between resistance values is key. A change in the resistance value of one resistor in relation to the other, due to some outside influence, can cause significant circuit errors. Resistors must, therefore, have low temperature sensitivity, high stability and tight tolerances.
In measurement and analysis equipment ambient temperature changes or localised heating effects that impact on resistor components can cause equipment measurement errors. The ideal solution is to employ resistors with a TCR of zero (the resistor maintains a fixed value as the temperature is varied) but a more practical method is to ensure both resistors change by the same magnitude (as a percentage) in response to the temperature change.
Comparing A Thick Film Resistor Network With Discrete Devices
A number of thick film resistor design and manufacturing factors can impact on the TCR of a resistor device but the most significant are:
The aspect ratio of the thick film resistor.
The ink composition used in the resistor print process.
Controlling these factors across different manufacturing lots can be a problem leading to potential wide variations in resistor TCR. It may be possible to select devices from the same lot with closely matched TCR but controlling how these devices are installed during the manufacturing process can be an issue.
There is always the possibility of human error leading to mixed lots. Discrete resistors that are not in close proximity on a system board can be impacted by differential heating effects and therefore variations in TCR. Unequal self heating of the resistor devices in the circuit can also be an issue.
To eliminate these problems one solution is to use a thick film resistor network device. Resistor devices are in close proximity and subject to the same external influences. The impact of any self heating is shared across devices. Resistors are manufactured at one time using the same materials eliminating lot by lot variations and on site manufacturing issues.
A wide selection of thick film resistor networks are available suitable for most requirements. When a standard device will not suffice then the best solution may be to work with an application specific resistor manufacturer to develop a custom resistor device. Matching TCR values on a resistor network can be a challenge, especially if there is a wide variation in the resistor values. A specialist manufacturer may deliver samples for analysis to help the system designer assess what is achievable and to request changes to resistor values to compensate for application issues.